Modern gas turbine engineering predominantly uses what are known as lean-burn premix burners. A very wide range of designs of lean-burn premix burners are known, for example from U.S. Pat. No. 4,781,030, EP 321 809, EP 780 629, WO 93/17279, EP 945 677 or WO 00/12936. These burners substantially work on the principle of introducing fuel into an airstream which has been greatly swirled up and in which this fuel forms a homogenous mixture with the combustion air. The ignition and flame stabilization are effected by the swirling flow breaking open at the burner exit, i.e. at the opening of the burner to the combustion chamber. It is preferable for these burners to be operated at a substoichiometric fuel/air ratio, typically with air/fuel ratios around 2. This prevents the formation of stoichiometric zones with hot spots in the flame, at which high levels of nitrogen oxides are produced, and the good premixing usually also results in a good level of burnup. These premix burners are often designed to operate in the region of the lean extinction limit, which restricts the operating range. Therefore, what are known are pilot stages or pilot burners, via which additional fuel is introduced into the combustion chamber in certain operating ranges, are used for operation with a fuel quantity which is below that required for stable premix operation.
Under certain unfavorable circumstances, all known premix burners may on occasion have a tendency to form thermoacoustic oscillations in the combustion chamber. These undesirable oscillations can be reduced firstly by suitable control of the fuel supply and of the fuel distribution and secondly by damping measures within the combustion chamber. For example, U.S. Pat. No. 5,685,157 has disclosed an acoustic damper for a combustion chamber which is formed by a plurality of resonating tubes which are in communication with the combustion chamber via a perforated plate. These resonating tubes serve as Helmholtz resonators which damp individual thermoacoustic oscillations depending on the size of the resonating volume. U.S. Pat. No. 5,431,018 also shows the use of Helmholtz resonators at a combustion chamber. In this document, an annular air duct for feeding cooling and combustion air into the combustion chamber, which is in communication with a resonator volume, is formed around the feedline for fuel leading to a combustion chamber. U.S. Pat. No. 6,164,058 has disclosed an arrangement for damping acoustic oscillations in a combustion chamber, in which the length of cooling passages formed at the combustion chamber wall is adapted in such a manner that these cooling passages have a minimal acoustic impedance at the location where the cooling air enters the burner. Some of this cooling air is then mixed with the fuel in the burner and at the burner exit is passed into the combustion chamber for combustion. Although Helmholtz resonators can achieve very high levels of damping, they can only do so in a very narrow frequency range, to which the resonance volume is tuned. They are particularly suitable for the damping of individual oscillations in the low-frequency range, in which the frequency separation between the undesirable oscillations is relatively great.
In modern gas turbine installations which operate with premix burners, however, higher-frequency oscillations which are close together may also occur in a wide frequency range as a result of what are known as combustion chamber pulsations, and these oscillations jeopardize the quality of the combustion process and also the structural integrity of the installations. Helmholtz resonators are relatively unsuitable for damping wide-band oscillations of this nature.